Method of making ductile iron-chromium alloys having alpha cold-rolled finish



Patented May 24, 1932 UNITED STATES PATENT OFFICE HARRY S. GEORGE, 013MASSAPEQUA, NEW YORK, ASSIGNOR TO ELECTED METALLURGI- CAL COMPANY, ACORPORATION OF WEST VIRGINIA mm'non OF MAKING DUCTILE IRON-CHBOMIUMALLOYS HAVING A COLD-ROLLED FINISH No Drawing. Original applicationfiled December 21, 1928, Serial No. 327,750. Divided and thisapplication filed March so, 1932. Serial No. 602,099.

My invention relates to methods of producing iron-chromium alloyarticles containing from 8% to 40% chromium and having a cold rolledfinish and a high ductility.

Steel which has been given a cold rolled finish is desired on account ofits characteristic bright, smooth and compact surface. It is possible tomake ordinary carbon steel and many alloy steels with this kind of asurface finish because the action of the surfacing rolls on the annealedmaterial does not destroy useful characteristics of the steel andbecause a desired stiffness is imparted to the steel in cold rolling.\Vhen iron-chromium alloys containing more than about 8% of chromium,such as stainless steel or rustless iron are given a cold rollingtreatment like that given to the carbon steels, the ductility of thematerial is practically destroyed or is so greatly reduced that thearticles have little or no practical utility.

The resistance of these alloys to corrosion depends to some extent uponthe physical structure of the alloys. Cold rolling changes the physicalstructure of the alloys and lowers their resistance to corrosion. Theduotility and resistance to corrosion may be restored by annealing butheretofore the characteristics of a cold rolled surface could not bepreserved because a thin oxide film was formed which could not beremoved without destroying the bright, compact and smooth surfacetexture.

It is among the objects of my invention to provide a method of producingductile ironchromium alloy articles having a cold rolled surface finishand to provide such articles free from surface oxides.

The best cold rolled surfaces are produced if the annealed articles arecold rolled when they are free from oxide. I take the annealed alloyarticle from which the oxide has been removed by an desired method andgive it a cold rolled nish. The rolling reduces its ductility to anundesirable degree. It is then given a heat treatment sufiioient torestore the ductility. The treatment may consist of heating to about 600to 800 C. A short heating such as a minute at this temperature isusually suflicient to restore the ductility to the cold rolled alloy buta longer heating or a higher temperature may be used in some caseswithout unduly oxidizing the surface or destroying the finish. However,

I prefer to heat the metal at such temperatures and for as short periodsof time as will suffice to restore the desired ductility and to form anoxide which can be removed by a method of cleaning hereinafter to bedescribed.

For most purposes, box annealing is used to restore ductility in orderto aifect the surface as little as possible and to produce a surfaceoxide of minimum thickness. I have obtained good results in boxannealing by heating in a reducing or a non-oxidizing atmosphere whichis free from hydrocarbon vapors, or which is at least not stronglyoxidizing. The referred method of box annealing consists in placing afew lumps of wood charcoal in the annealing box out of contact with themetal, heating to about 600 C. and holding the metal at this temperaturefor about ve minutes. A thin oxide is formed on the metal which ispractically immune to pickling acids.

To clean the oxide from the article, I use the method described in myco-pending application entitled Method of removing oxide coatings frommetals, Serial No. 327 ,756 filed December 21, 1928. The method consistsof treating the metal in an alkali bath which preferably contains anoxidizing agent. The

surface oxide is then removed in an acid bath. For the present purposesof illustration, satisfactory results are obtained by using 20% sodiumhydroxide solution at about 70 C. in which chlorine has been bubbleduntil a cloud of oxygen appears in the solution gives the best resultsbut less concentrated acid baths may be used. It is generallyadvantageous to have not more than 1% hydrochloric acid present whenmixed acid is used. For cleaning iron-chromium alloys containing 8% to20% .chromium the concentration of the mixed nitric and hydrochloricacids may be from 1% to 10%. For cleaning rustless iron containing 16%to 18% chromium the best results are obtained by using a 3% nitric acidbath containing less than Ultimate tensile strength, pounds per squareinch Per cent A. Box-annealed and not cold-rolled 75, 000 B. Cold-rolled110, 000 C. Cold-rolled, heat treated and cleaned..." 78, 000

Comparison of examples A and B shows that cold rolling practicallydestroyed the ductility of the article. A comparison of examples A and Cshows that the'finished article produced in accordance with theforegoing description had ,ductility throughout about the same as a hotworked annealed article of the same composition.

Various modifications of my invention may be made such as removing allthe oxide after the article is annealed. Therefore, I desire to belimited only by the prior art and by the invention as defined in theannexed claims. 7

This application is a division of my copending application Serial No.327,750, filed December 21, 1928.

I claim as my invention:

1. The method of producing a ductile article consisting of an alloycomprising iron and about 8% to 40% chromium which meth od comprisescold rolling an oxide-free surface, heating in a. non-oxidizing orreducing atmosphere to a temperature sufficient to impart ductility tothe article, immersing in a solution of sodium hydroxide in thepresence' of an oxidizing agent, and then in a solution of mixedhydrochloric and nitric acids of a concentration insufilcient to destroythe cold rolled surface.

2. The method of producing a ductile irom chromium article containingabout 8% to 20% chromium which comprises hot rolling, annealing,removing the surface oxide, cold rolling so as to give a cold rolledfinish, heating to 600 C. to 800 C. to restore ductility to the article,treating at about C. with a 15% to 30% caustic soda solution to which asmall amount of chlorine has been added and then treating in a dilutesolution containing 1% to 10% mixed nitric and hydrochloric acids, saidmixture containing not more than 1% hydrochloric acid.

3. The method of producing a ductile article with a cold rolled finishconsisting of an iron-chromium alloy containing 8% to 40% chromium whichcomprises cold rolling the article to give it a smooth compactsurfacelayer, heating in a closed container in the presence of but outof contact with charcoal, said heating being carried on for a time andto a temperature sufiicient to restore-ductility throughout'the article,and then removing surface oxides by immersing in an alkali bath and thenin an acid bath of a concentration insufficient to destroy the coldrolled luster which is uncovered by the action of said alkali bath andsaid acid bath.

4. The method of producing a; ductile article with a cold rolled finishconsisting of an iron-chromium alloy containing 8% to 40% chromium whichcomprises cold rolling the article to give it a smooth compactsurfacelayer, heating to 600 to 800 C. in a closed container in thepresence of but out of contact with charcoal, said heating being for atime suflicient to restore ductility throughout the article, and thenremoving surface oxides by immersing in an alkali bath and then in anacid bath of a concentration insufiicient to destroy the cold rolledluster which is uncovered by the action of said alkali bath and saidacid bath.

5. The method of producing a ductile article consisting of aniron-chromium alloy which contains about 8% to 40% chromium whichcomprises cold rollin an annealed article which is free from sur aceoxides, heating to temperatures suflicient to restore duc tility to thearticle, said alloy being out of contact with carbon and the atmospherearound the alloy being free from hydrocarbon gases which producecarbon-containing surface layers on the alloy and from oxidizingscaleorming gases, immersing in an alkali solution, and immersing in anacid solution which has a concentration insuflicient to destroy the coldrolled finish.

6. The method of producing a ductile article consisting of aniron-chromium alloy which contains about 8% to 40% chromium whichcomprises cold rolling an annealed art1cle so as to produce a coldrolled finish, heat ng to temperatures suflicient to restore ductihty tothe article, said alloy bein out of contact with carbon and the atmoseres around the alloy being free from by rocarbon gases which producecarbon-containing surface la ers on the alloy and from oxidizingscaleorming gases, immersing in an alkali solution, and immersing in anacid solution which has a concentration insufiicient to destroy the coldrolled finish.

7. The method of producing a ductile article consisting of aniron-chromium alloy which contains about 8% to 40% chromium whichcomprises cold rolling an annealed article so as to produce a coldrolled finish, heating the temperatures sufiicient to restore ductilityto the article, said alloy being out of contact with carbon and theatmospheres around the alloy being free from hydrocarbon gases whichproduce carbon-containing surface lafyers on the alloy and fromoxidizing scaleorming gases, immersing in an alkali solution, andimmersing in a solution of nitric and hydrochloric acids which containsmore nitric than hydrochloric acid and has a concentration insufiicientto destroy the cold rolled finish.

8. The method of producing a ductile article consisting of aniron-chromium alloy containing 8% to 40% chromium which comprises coldrolling the article to give it a smooth compact surface-layer, heatingto 600 to 800 C. in a closed container in the presence of but out ofcontact with charcoal, said heating being for a time sufficient torestore ductility throughout the article, and then subjecting thearticle to an alkali bath to which chlorine has been added and to anacid bath of a concentration insufiicient to destroy the finish of thesmooth compact surface layer.

9. The method of producing a ductile article consisting of aniron-chromium alloy containing 8% to 40% chromium which comprises coldrolling the article to give it a smooth, compact surface-layer, heatingto 600 to 800 C. in a closed container in the presence of but out ofcontact with charcoal,

- said heating being for a time suflicient to restore ductilitythroughout the article, and then subjecting the article to an alkalibath to which chlorine has been added and to an acid bath, containing adilute solution of mixed nitric and hydrochloric acids of aconcentration insufiicient to destroy the finish of the smooth compactsurface layer.

In testimony whereof, I afiix my signature.

HARRY S. GEORGE.

